The present invention relates to an analogue display electronic timepiece using a stepping motor, and more particularly to an improvement for controlling the output and power consumption of the stepping motor at a constant level even if the supply voltage of a power source is higher than the optimum voltage and even if the supply voltage and the internal resistance of the power source vary.
Before describing the invention, a typical electronic timepiece using a stepping motor conventionally used will be illustrated.
FIG. 1 shows a block diagram of the conventional electronic timepiece. A quartz crystal resonator 1 is connected to an oscillating circuit 2 generates an oscillatory signal of 32768 Hz frequency. The oscillatory signal is fed to a frequency divider 3 and divided into one second signals by a flipflop comprised of 15 stages. Subsequently, a wave shaping circuit 4 composes driving pulse signals necessary for driving a stepping motor 6. A driving circuit 5 flows current in the stepping motor 6 according to the driving pulses produced from the wave shaping circuit 4.
FIG. 2(a) shows an overall perspective view of the stepping motor, where reference numeral 11 denotes a stator, 12 denotes a rotor and 13 denotes a coil. The construction of this stepping motor is the same as the stepping motor used in the embodiments of the present invention.
FIG. 2(b) shows a voltage waveform of the driving pulses applied across the coil 13 from the driving circuit 5. As shown, the driving pulses comprise alternate polarity pulses of one second period and having a pulse width of 6.8 msec.
All the circuits shown in FIG. 1 are fabricated as an IC and the power is supplied from a power source 7. The power source 7 is generally a silver battery which shows a plain discharging characteristic of 1.5 V up to nearly the end of the battery life and therefore the operations of the circuits and the stepping motor are stable. The supply voltage is easily detected by adding a battery life displaying device which detects the battery voltage at the end of the battery life when the supply voltage begins to drop. Therefore it is not necessary to take into account the voltage variation when designing the stepping motor.
On the other hand, in case a peroxide silver battery is used for the power source, since the supply voltage of the peroxide silver battery is 1.8 V just after it is manufactured, the operation of the stepping motor is unstable.
In order to eliminate the above mentioned defect, the battery capacity is reduced to 1.5 V by a silver treatment process during the manufacture of the peroxide silver battery. However, the feature of the peroxide silver battery, i.e., the feature that the battery capacity is large with respect to its volume, is not made the best use of.
Although a lithium battery is advantageous in view of its high reliability and high energy density, the discharging characteristic is exceedingly disadvantageous.
FIG. 3(a) shows the discharging characteristic of the lithium battery. As shown in FIG. 3(a), a voltage of 3 V in the beginning drops to 2.7 V after a fixed time and remains at 2.7 V for a while and thereafter the voltage reduces gradually. It is exceedingly difficult to drive the conventional stepping motor stably by the lithium battery having the above mentioned characteristics. Further, since the current and output torque are also influenced by the voltage, it is difficult to drive the stepping motor in the range of the variable voltage exhibited by the lithium battery. Furthermore, in the case that the stepping motor for 3 V is used with the conventional power, it is necessary to wind the motor coil with a thin wire in order to increase the coil resistance and this results in an increased manufacturing cost.
FIG. 3(b) shows a variation of the supply voltage in the case that a silver battery or peroxide battery is used as a secondary battery and charged by a solar battery. In this case the supply voltage constantly varies between 1.57 V and 1.8 V by repetition of charging and discharging. And since the performance of the stepping motor becomes unstable as described above, a controlling method as shown in block form in FIG. 4 has been devised.
FIG. 4 is a block diagram showing a supply voltage detecting circuit 9 and a controlling circuit 8 which varies the pulse width of the driving pulses according to the voltage variation in order to shorten the driving pulse width in the case the supply voltage is high. In other respects, the circuitry is like that shown in FIG. 1.
Several characteristics of the stepping motor versus voltage in the case where the driving pulse width is varied are shown in FIGS. 5(a), 5(b), 5(c) and 5(d).
FIGS. 5(a), 5(b), 5(c) and 5(d) respectively show a mean current, an output torque at a center wheel and pinion, an efficiency and a peak current value versus the voltage.
However, in this prior art method of varying the driving pulse width of the stepping motor, there is an operating range where the operation of the stepping motor is unstable by the combination of a specific voltage and pulse width as shown in FIGS. 5(a) and 5(b) (in this embodiment from about 2.3 to 2.7 V), and the desired output torque cannot be obtained in this voltage range. While the efficiency decreases in accordance with the voltage as shown in FIG. 5(c) and the peak current increases in accordance with the voltage as shown in FIG. 5(d), the design of the power source battery and the IC are restricted. Thus there are a number of drawbacks in the conventional driving method for a practical use.